Methods for packaging image sensitive electronic devices

ABSTRACT

The invention provides methods for packaging for electronic devices that are light or other radiation-sensitive, such as image sensors including CCD or CMOS chips. In one embodiment of the invention, an image sensor package is assembled by surrounding a chip with a barrier of transfer mold compound and covering the chip with a transparent lid. In another embodiment of the invention, the perimeter area of a chip, including interconnections, is encapsulated with a liquid dispensed epoxy, and a transparent lid is attached. In yet another embodiment of the invention, a unitary shell of entirely transparent material is used. In yet another embodiment of the invention, a substrate-mounted chip and a transparent lid are loaded into a transfer mold that holds them in optimal alignment. The transfer mold is then filled with molding compound to encapsulate the chip and interconnections, and to retain the transparent lid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/370,674,filed Feb. 21, 2003, pending, which is a divisional of application Ser.No. 10/164,077, filed Jun. 4, 2002, now U.S. Pat. No. 6,906,403, datedJun. 14, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electronic devices that aresensitive to light or other forms of radiation. More particularly, thepresent invention relates to packages with transparent coverings forenclosing image sensors and methods for their assembly.

2. State of the Art

Solid-state image sensors, for example, charge coupled devices (CCDs) orcomplementary metal-oxide semiconductor (CMOS) chips, are increasinglyin demand for use with electronic devices such as digital cameras.Conventionally, these sensors have been packaged for use by mountingthem to a substrate and enclosing them within a housing assembly. Thehousing assembly incorporates a transparent lid to allow light or otherforms of radiation to be received by the sensor. The lid may be a flatwindow or shaped as a lens to provide optical properties. The substrateand housing are often formed from a ceramic material, and the lid isglass or a similar transparent substance attached to the housing by anadhesive. Due to the materials and structure involved, this packagingtechnique may be expensive and difficult to manufacture. Further,growing desire for portable electronic devices that will stand up toextreme environments raises concerns of durability and size.

In order to better meet large-scale production requirements, variousmethods have been developed in attempts to simplify the construction ofimage sensors. Examples include U.S. Pat. No. 6,266,197 to Glenn et al.,which discloses fabricating multiple sensor housings at one time bymolding window arrays. U.S. Pat. No. 6,072,232 to Li et al. discloses aplastic package with an embedded frame for reducing material costs. U.S.Pat. No. 5,811,799 to Wu discloses a simplified substrate that is aprinted wiring frame with a wall erected thereon. While these and otherdesigns have been of some benefit, they still involve a number ofspecialized parts requiring multiple, sometimes precision, steps forassembly. Issues concerning material costs and part complexity remain.

Accordingly, a need exists for improved image sensor packaging that issimple to assemble and cost effective, while being of durable and lightconstruction.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, improved packages for imagesensors and methods for their assembly are disclosed. Embodiments of thepresent invention are directed to image sensor chips or similar light orother radiation-sensitive electronic components mounted on a carriersubstrate. The chips are mounted to the carrier substrate in aconventional manner with epoxy or a tape lamination process andelectrically connected to substrate conductive elements with wire bondsor other suitable interconnections. The chips are enclosed within aprotective barrier including a transparent covering which acts as awindow or lens. If desired, an array of chips may be mounted on a singlelarge substrate and the substrate later sectioned to form individualimage sensor packages. Once assembled, the image sensor packages may beincorporated into electronic devices by attachment with electricalconnections on the carrier substrate. The materials used are inexpensivewhile providing durable, lightweight and simple construction.

In one exemplary embodiment of the invention, an image sensor package isassembled by surrounding a chip with a barrier of transfer mold compoundsuch that the chip and its interconnections are left exposed. Thisexposed area coincides with the outer perimeter of a transparent lid. Anadhesive of a known, suitable type is applied to the perimeter of thelid, which is then positioned at an optimum distance from the chipwithin the exposed area. If the adhesive requires curing, the adhesivemay be activated to secure the lid to the barrier of mold compound.

In another exemplary embodiment of the invention, the perimeter area ofa chip, including interconnections such as wire bonds and bond pads, isencapsulated with a liquid dispensed epoxy or other liquid dispensednonconductive material. A transparent lid placed on top of the chip isretained by adhesion with the epoxy that encapsulates the chipperimeter. The lid may be placed directly on the chip, or a gasket maybe used, if necessary, to prevent epoxy from seeping into the areabeneath the lid due to capillary action. Once the lid is attached, theassembly may be further encapsulated by a transfer mold compound or apot mold compound for increased protection if desired.

In yet another exemplary embodiment of the invention, chip encapsulationis accomplished with a unitary shell of entirely transparent material.The shell may be formed by molding a transparent compound directlyaround the chip or by attaching a preformed shell to the carriersubstrate with an adhesive. The clearness or transmissivity of the shellsurface may be improved by using a very smooth mold surface or bypolishing the shell after it is molded.

In yet another exemplary embodiment of the invention, asubstrate-mounted chip and a transparent lid are loaded into a moldtooling element, such as a transfer mold configured to hold them inoptimal alignment. The transfer mold is then filled with moldingcompound to encapsulate the chip and interconnections, and to secure thetransparent lid in place. A conformal film may be disposed between thesurface of the mold cavity and the transparent lid to act as a gasketfor controlling mold compound flash on the surface of the transparentlid.

As a further variation, any of the above-described exemplary packageembodiments may be used with one or more electronic components of astacked multi-chip module (MCM). The chips are mounted to a carriersubstrate and to each other in a conventional manner with epoxy or atape lamination process. The chips are electrically interconnected toeach other and to the substrate conductive elements with wire bonds orother suitable interconnections.

Other and further features and advantages will be apparent from thefollowing descriptions of the various embodiments of the presentinvention when read in conjunction with the accompanying drawings. Itwill be understood by one of ordinary skill in the art that thefollowing are provided for illustrative and exemplary purposes only, andthat numerous combinations of the elements of the various embodiments ofthe present invention are possible.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which illustrate what is currently considered to be thebest mode for carrying out the invention:

FIGS. 1A-1E are schematic sectional side views showing an exemplarymethod of formation of an image sensor package, wherein encapsulationincludes surrounding a chip with a barrier of transfer mold compound andattaching a transparent lid to the barrier with adhesive applied to theperimeter of the lid;

FIGS. 2A-2E are schematic sectional side views showing an example offorming an image sensor package using the same method as in FIGS. 1A-1E,except that the barrier of transfer mold compound is formed on thecarrier substrate prior to mounting the chip;

FIG. 3 is an alternative partial schematic sectional side view of theimage sensor package formed by the methods of FIGS. 1A-1E or FIGS.2A-2E, wherein a ledge is formed on the inner perimeter of the moldedbarrier to support the transparent lid;

FIGS. 4A-4F are schematic sectional side views showing an exemplarymethod of forming an image sensor package, wherein encapsulationincludes covering the perimeter area of a chip, includinginterconnections such as wire bonds and bond pads, with a liquiddispensed epoxy;

FIG. 5 is an enlarged schematic sectional view of the region II of FIG.4F showing a layer of molding compound covering the epoxy and theperimeter area of the chip, and further retaining the edges of atransparent lid;

FIGS. 6A-6D are schematic sectional side views showing an exemplarymethod of forming an image sensor package, wherein encapsulationincludes covering a chip with a unitary shell of entirely transparentmaterial;

FIG. 7 is an alternative schematic sectional side view of the imagesensor package formed by the method of FIGS. 6A-6D, wherein a preformedshell is used to cover the chip;

FIGS. 8A-8E are schematic sectional side views showing an exemplarymethod of forming an image sensor package, wherein encapsulationincludes molding a chip and transparent lid together within a transfermold;

FIGS. 9A-9E are schematic sectional side views showing a method offorming an image sensor package as in FIGS. 8A-8E, wherein multipleelectronic components in the form of a chip stack are encapsulated; and

FIG. 10 is an enlarged schematic sectional view of the region IV of FIG.9E showing a layer of molding compound covering the perimeter area ofthe chip stack, and further retaining the edges of the transparent lid.

DETAILED DESCRIPTION OF THE INVENTION

Generally, the present invention includes methods for assemblingpackaged image sensors that include solid-state devices, such as CCD orCMOS chips, to receive an image of reflected light or otherelectromagnetic radiation from one or more objects. It should beunderstood, however, that the packages and other methods of the presentinvention would also work well for enclosing other types of light orother radiation-sensitive electronic components such as, for instance,erasable programmable read-only memory chips (EPROMs).

Referring to the accompanying drawings, wherein similar features andelements are identified by the same or similar reference numerals, thevarious embodiments of the image sensor packages are formed on a carriersubstrate 2. Carrier substrate 2 includes conductive elements 1 (e.g.,traces) having first ends with terminal pads 5 for electrical connectionwith components, such as a chip 4, on carrier substrate 2. Wire bonds 8are shown as the intermediate conductive elements 1 that electricallyconnect bond pads 5′ of chip 4 and corresponding terminal pads 5 on theupper surface of carrier substrate 2, but other suitableinterconnections, including flip-chip or conductive tape-automatedbonding (TAB) elements carried by a dielectric, polymeric film, may beused when compatible with the structures of chip 4 and carrier substrate2. The second ends of the conductive elements 1 terminate in attachmentpads 3 for connecting carrier substrate 2 to the surface of a largerassembly, such as a printed circuit board. Carrier substrate 2 may beconstructed of plastic, such as thermoplastic and thermosettingplastics, which is less expensive and lighter than the ceramicsubstrates typically used for image sensors. Other common substratematerials such as FR-4 or BT would be suitable as well.

Turning to FIGS. 1A through 3, a first embodiment of the presentinvention is illustrated. An optical sensor in the form of a chip 4 ismounted on carrier substrate 2 with an adhesive layer 6 formed of epoxyor provided by an adhesive-coated tape in a lamination process as knownin the art. Chip 4 is then electrically connected to carrier substrate 2by wire bonds 8. Next, a protective barrier 10 is formed around thechip, such as by known transfer molding, pot molding, or injectionmolding processes, by photolithographic processes, by stereolithographicprocesses, or as otherwise known in the art. As shown in FIGS. 1A-1E,molded protective barrier 10 is in the shape of a wall surrounding acentral exposed area containing chip 4 and its interconnections,including wire bonds 8. Once protective barrier 10 is formed, atransparent lid 12, having substantially the same size as the outerperimeter of the central exposed area, is lowered into the protectivebarrier 10. Transparent lid 12 is positioned at a distance from the chip4 that provides optimal performance. For instance, if transparent lid 12is shaped as a lens in order to possess certain desired opticalcharacteristics, the transparent lid 12 may be located at a specificfocal point to ensure optimal image sensing. Positioning may beaccomplished by contacting electrical probes 14 to the attachment pads 3on the lower surface of the carrier substrate 2 to monitor for a desiredsensor output while lowering transparent lid 12 within protectivebarrier 10. Alternatively, as shown in FIG. 3, if a set optimal distancefor placement of transparent lid 12 from chip 4 is known, a ledge 16 maybe formed on the inner perimeter 11 of protective barrier 10 to supporttransparent lid 12 at that distance. After transparent lid 12 has beenlocated in the proper position, an adhesive coated on a perimeter 13 oftransparent lid 12 or at an appropriate location of protective barrier10 secures transparent lid 12 into place. The adhesive may comprise apressure-sensitive adhesive or a so-called “activated adhesive.” Theterm “activated adhesive” refers to materials that do not completelyadhere or cure until acted upon by a specific initiating agency.Examples include UV curing, thermosetting or chemically activated epoxy.Chip 4, bond pads 5′ thereof, and the intermediate conductive elements(e.g., wire bonds 8) that are secured to terminal pads 5, areeffectively sealed in an area surrounded by protective barrier 10 andunder transparent lid 12.

Protective barrier 10 is constructed of, for example, molding compoundor a similar encapsulant material formed onto carrier substrate 2 usinga transfer mold. As shown in FIGS. 2A-2E, under certain circumstances itmay be desirable to form protective barrier 10 on carrier substrate 2prior to mounting chip 4 to carrier substrate 2. This may reducepossible contamination or damage to chip 4 resulting from the process offorming protective barrier 10. Further, it would allow the formation ofprotective barrier 10 to be performed outside of a clean roomenvironment which might otherwise be required if chip 4, which is highlysensitive to air-born contaminants, was already attached.

A second embodiment of the present invention is illustrated in FIGS.4A-4F and 5. Image sensor chip 4 is mounted to carrier substrate 2 andconnected by wire bonds 8 in the same manner as described above. Aliquid epoxy 18 or other nonconductive adhesive material is thendispensed around the perimeter 7 of chip 4 to encapsulate wire bonds 8and corresponding bond pads 5′ on chip 4 and terminal pads 5 on carriersubstrate 2. Transparent lid 12 is placed over chip 4, and contact oftransparent lid 12 with liquid epoxy 18 that encapsulates the chipperimeter 7 provides an adhesive interface to retain transparent lid 12in position over chip 4. The liquid epoxy 18 is then cured to securetransparent lid 12 in place. Transparent lid 12 may be placed directlyagainst the surface of chip 4, or a gasket 9 may be inserted between thetwo, if desired, to prevent liquid epoxy 18 from seeping into the areabeneath transparent lid 12 due to capillary action. In the case wheretransparent lid 12 is formed as a lens, gasket 9 may also supporttransparent lid 12 at an optimal distance from chip 4, much like ledge16 in FIG. 3 of the previously described embodiment.

After attachment of transparent lid 12, the assembly is suitable for useas an image sensor package without any further encapsulation. In somecases, however, it may be desirable to reinforce the assembly withadditional material in the form of molding compound. FIGS. 4E-4F show alayer 20 of molding compound applied to the assembly using a transfermolding process, although other known and suitable techniques (e.g., potmolding, injection molding, photolithography, stereolithograpy, etc.)may be used to form layer 20. As can be seen in FIG. 5, layer 20 coversliquid epoxy 18 that encapsulates wire bonds 8 and the perimeter 7 ofchip 4 and, further, retains the position of transparent lid 12 relativeto chip 4.

FIGS. 6A-6D and 7 illustrate a third embodiment of the presentinvention. Chip 4 is mounted and connected to carrier substrate 2 as inthe previously described embodiments. Next, a shell 22, formed entirelyof a transparent material, may be added to the assembly forencapsulation of chip 4 and its associated interconnections, which, inthe example of FIGS. 6A-6D, are wire bonds 8. The shell 22 may be formedby molding a transparent compound directly around chip 4 or, as in FIG.7, attaching a preformed shell 22′ to carrier substrate 2 with anadhesive.

In the case where shell 22 is formed directly around chip 4, a clearcompound will be molded onto carrier substrate 2. Clear epoxy or otherinexpensive resin-type materials, like polystyrene and silicon, may beused for the clear compound so long as they harden into a durable andhighly transparent structure. Once formed, shell 22 acts both as aprotective encapsulant for chip 4 and its interconnections and as atransparent lid for image sensing. An even, untextured surface improvesthe clearness or transmissivity of shell 22, which is important foroptimal image sensing. This may be accomplished during the moldingprocess by using a mold cavity made with one or more very smoothsurfaces overlying the face of the chip 4. The mold cavity may also beused to determine optical qualities by imparting lens shapes or focusingsurface features on the shell. Alternatively, the surface of shell 22may be polished by known processes after molding to carrier substrate 2.The option of molding shell 22 directly around chip 4 offers theadvantage of securing all the connections within a highly protectivesolid structure.

Turning to FIG. 7, when preformed shell 22′ is used rather than moldinga shell 22 directly around chip 4, the shell 22′ may be spaced apartfrom chip 4, forming a cavity 24 thereover, instead of comprising asolid structure on the surface of chip 4. An adhesive layer 26, such asepoxy or tape, may also be required for attachment at the interfacebetween preformed shell 22′ and carrier substrate 2. Enclosing chip 4 inthis manner advantageously allows preformed shell 22′ to later beremoved if repair or replacement of parts in the image sensor package isnecessary. If removal is not a concern, after preformed shell 22′ isattached, cavity 24 may be filled in with a clear compound 25 like thatused for molding so as to strengthen the package.

Another embodiment of the present invention is illustrated in FIGS. 8Athrough 10. After mounting and connecting chip 4 to carrier substrate 2,the assembly is loaded into a mold tooling 28. Transparent lid 12 isalso loaded into mold tooling 28 so that transparent lid 12 and chip 4are held into alignment and position for proper image sensing. Thecavity 29 (not shown) of mold tooling 28 is then filled with moldingcompound so as to create a layer 30 covering the area around theperimeter 7 of chip 4 and to retain transparent lid 12 while leaving itsupper surface exposed. The encapsulation for the image sensor package isthereby completed. In order to prevent molding compound flash onto thetop surface of transparent lid 12 during the molding process, aconformal film 32 may be applied between the interior surface of moldtooling 28 and transparent lid 12. This produces a gasketing effect andprevents molding compound intrusions.

FIGS. 9A-9E shows a variation of the fourth embodiment of the presentinvention, wherein multiple electronic components 4, 4′, 4″ in the formof a multi-chip module 34 are enclosed within the package. Components 4,4′, 4″ may be mounted to carrier substrate 2 and to each other withepoxy or adhesive-coated tape 6, 6′, 6″, and interconnected with wirebonds 8, 8′, 8″ or other suitable interconnections. Of course,multi-chip module 34 may be comprised of any number of components aslong as the image sensor chip 4 is on top of the chip stack or in aposition to be clear of obstruction. The encapsulation process thencontinues as previously described, with carrier substrate 2 and attachedmulti-chip module 34 being loaded into mold tooling 28 for alignment andmolding with transparent lid 12. As can be seen in FIG. 10, layer 30encapsulates the perimeter area and interconnections of multi-chipmodule 34, and retains transparent lid 12. Although an example ofpackaging multi-chip module 34 has been illustrated only in terms of thepresent transfer mold tooling embodiment, it should be clear that thefirst three encapsulating embodiments discussed above would also workwell for enclosing a multi-chip module 34.

In all of the embodiments, after the encapsulation step the package mayproceed through other conventional assembly steps. This includesattachment of solder balls 36, or other discrete conductive elementssuch as conductive or conductor-filled epoxy pillars, columns or bumps,on the bottom of carrier substrate 2 to form an easily mountable BGApackage. Moreover, if the packaging has been assembled as an array ofchips attached to one large substrate, it may be cut or otherwisedivided into pieces that each comprise a single image sensor package.

It is also within the scope of the present invention that certainaspects of one of the described embodiments might be used in thefabrication of another of the described embodiments. The method ofmolding transparent shell 22 around image sensor chip 4 with clearcompound, for instance, may be used for encapsulation in otherembodiments. In the first embodiment, rather than using transparent lid12 to cover the central exposed area of protective barrier 10, a clearcompound may simply be filled into the central exposed area, with theupper surface of the clear compound being formed into the desired windowor lens shape. Further, in the embodiments where transparent lid 12 isplaced directly against the sensing surface of chip 4, the focal pointfor image sensing is constant and will be optimized based only onshaping when the form of a lens is required. In any of thoseembodiments, a transparent lid of clear compound directly molded ontochip 4 may be advantageous.

All of the above-illustrated embodiments of the present inventionprovide image sensor packages that are constructed of low cost materialand require simple methods of assembly. The packages are also durableand lightweight, making them highly desirable for use in increasinglydemanding operating conditions. Although the present invention has beendepicted and described with respect to the illustrated embodiments,various additions, deletions and modifications are contemplated from itsscope or essential characteristics. Furthermore, while described in thecontext of image sensor packaging, the invention has utility forpackaging of all components that are sensitive to light or otherradiation and require a transparent surface for exposure. The scope ofthe invention is, therefore, indicated by the appended claims ratherthan the foregoing description. All changes which come within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

1. A method for assembling a package containing an electronic devicesensitive to light or other radiation comprising: mounting saidelectronic device on a carrier substrate having at least one conductivetrace thereon, said at least one conductive trace having a first end anda second end; providing at least one electrical interconnection betweensaid electronic device and said first end of said at least oneconductive trace; loading a transparent lid into a mold tooling element;loading said carrier substrate with said electronic device and said atleast one electrical interconnection into said mold tooling element sothat a surface of said electronic device is aligned against saidtransparent lid; filling said mold tooling element with moldingcompound; forming a molded layer on said carrier substrate toencapsulate said at least one electrical interconnection and a perimeterof said electronic device; and retaining said transparent lid with saidmolded layer.
 2. The method according to claim 1, further comprising:applying a layer of conformal film between an inside surface of saidmold tooling element and said transparent lid prior to said filling saidmold tooling element.
 3. The method according to claim 1, furthercomprising selecting said electronic device to be an image sensor. 4.The method according to claim 1, further comprising selecting saidelectronic device to comprise a solid-state device selected from a groupconsisting of a charge coupled device, a complementary metal-oxidesemiconductor and an erasable programmable read-only memory.
 5. Themethod according to claim 4, further comprising fabricating saidelectronic device as a chip stack.
 6. The method according to claim 1,further comprising forming said carrier substrate of a material selectedfrom a group consisting of plastic, FR-4 and BT.
 7. The method accordingto claim 6, further comprising: attaching at least one discreteconductive element to said second end of said at least one conductivetrace of said carrier substrate.
 8. The method according to claim 7,wherein a plurality of said packages, each containing an electronicdevice, are formed onto a single carrier substrate and furthercomprising: dividing said single carrier substrate into separate pieces,each of said pieces comprising one of said packages.